Apparatus for continuous gas diffusion



July 6, 1965 J. H. BECK APPARATUS FOR CONTINUOUS GAS DIFFUSION 4Sheets-Sheet 1 Filed July 31, 1961 INVEN TOR.

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July 6, 1965 Filed July 31, 1961 United States Patent 3,193,267APPARATUS FOR CONTINUQUS GAS DIFFUSON Jacob Howard Beck, Waban, Mass,assignor to BTU Engineering Corporation, Waltham, Mass, a corporation ofDelaware Filed July 31, 1961, Ser. No. 127,932 14 Claims. (Cl. 263-37)This invention relates to gas diffusion and more particularly to a gasdiffusion furnace for use by the semiconductor industry.

One of the chief methods of making semiconductors is by gaseousdifiusion of impurities. In this type of construction, a crystal ofsemiconductor material such as silicon and'a source, i.e., a mass ofimpurity such as antimony, are sealed together in a quartz tube and thecomplete assembly heated in a furnace to a very high temperature in theorder of about 1200 C. At this temperature, the impurity is in the formof a gas which diffuses into the surface of the crystal, thereby formingP- or N-type layers. The reaction time is usually in the range of 16 to36 hours. An advantage of this technique is the ability to form verylarge, fiat junctions, i.e., diffused layers, of controlled thicknesses.However, heretofore it has not been feasible to execute this techniqueon a continuous basis because of the inability to achieve strictdiffusion control, a prerequisite to obtaining diffused layers ofprecise thicknesses. Basically, the extent of diffusion depends on threefactors: (1) the concentra tion of impurity, (2) heating time, and (3)temperature. The first two variables are easily controlled, buttemperature presents a problem. Not only does the rate of diffusion varywith temperature, but in addition, the rate of change is not linear buta complex exponential function. Hence a fiat temperature profile isrequired. Because of this temperature problem and also because of thebelief that it is difficult to maintain the atmosphere free of undesiredcontaminants, continuous furnaces have been considered unfea-sible forgaseous diffusion and batch furnaces have been used instead. Someconsideration has been given to making a continuous diffusion furnacewherein the quartz tube containing the semiconductor crystal and thesource is moved along its own longitudinal axis through the furnace.However, this approach is not feasible. Proper diffusion requires thatthe concentration of the gaseous impurity be maintained at apredetermined value. This is achieved by precisely controlling thetemperature to which the source is heated. If the temperature varies,the concentration of the gaseous impurity will go up or down and,therefore, the final product will be affected. An attempt to move thequartz tube axially through any sort of continuous furnace at one pointor another will cause a change in the temperature of the source andcrystal. Thus, for example, when the tube is entering the furnace, somevapor will enter the crystal before the crystal has attained fulldiffusion temperature, but the rate of diffusion at this point would besmaller than desired. On the other hand, as the tube progresses towardthe exit end of the furnace, the source will be overheated. If thesource is an alloy, overheating will decompose it. If the source is asingle element, overheating will produce an excessive concentration ofimpurity vapor and also a difli'erent partial pressure. In either case,the overheating will cause uncontrolled and uncomputable diffusion.

Although batch furnaces do permit precise control of the thickness of adiffused layer, they have certain important drawbacks such as (1) lowproduction, (2) excessive handling, (3) high labor requirements, and (4)furnace deterioration. In normal batch furnace operation, the furnace iscooled down between batches. This 3,l3,2? Patented July fi, 1965repeated cooling causes rapid deterioration of the furnace, particularlyof the heating elements. In addition to cost of replacement, heaterdeterioration causes uneven heating which in turn prevents normaloperation.

Accordingly, the object of this invention is to provide a diffusionfurnace which is capable of continuous operation with precise diffusioncontrol.

A further object s to provide a novel method of diffusing impuritiesinto the surface of semiconductor crystals.

A more specific object of this invention is to provide a continuousdiffusion furnace which provides quality control equal to that of abatch furnace yet is free of the disadvantages of .a batch furnace. In acontinuous diffusion furnace embodying the present invention, diffusionis carried out on a precisely controlled temperature flat, therebyyielding diffused layers of precise thicknesses. This is achieved byproviding separate heating zones for the source and crystal and movingthem solely through their own zones. The source and crystal aresupported in a heating tube which is mounted at to its path of movement,simultaneously through two heating zones. As viewed in cross-section, afurnace constructed according to the present invention comprises twoseparate furnace sections, appropriately described as source heater anddiffuser heater, which are precisely controlled at differenttemperatures. A conveyor transports successive quartz tubes through thefurnace, each quartz tube containing a source of impurity to bevaporized and a crystal which is to be doped by diffusion of theimpurity. The impurity is vaporized in the source heater and diffusesinto the crystal in the diffusion heater. In the practice of thisinvention, the quartz tubes may be closed off by a hermetic seal or theymay be adapted for circulation of an inert carrier gas.

Other objects and many of the attendant advantages of the presentinvention will be readily appreciated as the invention becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a side View of a continuousdiffusion furnace embodying thepresent invention;

FIG. 2 is :an end view of the same furnace;

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 1;

FIG. 4 is an enlarged fragmentary side view, partly in section, of thesame furnace;

FIG. 5 is a schematic plan view of the same furnace as employed forclosed tube operation;

FIG. 6 is a schematic plan view showing the invention as applied to opentube'operation; and

FIG. 7 is a schematic plan view showing how the invention is applied toa double diffusion process.

Referring now to FIGS. 1 and 2, the furnace is mounted on upper andlower metal supporting frames. The upper frame is made of uprights 2,crossbars 4, and longitudinal beams 6. The lower frame is made up ofuprights 8, crossbars 10, and beams 12. The furnace itself comprises atop body section 16 attached to the undersides of crossbars 4 and abottom body section 18 supported by a plurality of beams 20 mounted oncrossbars 10.

As seen in FIGS. 3 and 4, each furnace section comprises an outer metalcase 24 by which it is attached to the frame and a ceramic liner 26attached to the metal case. The top and bottom furnace sections aremirror images of each other, having opposed side edge flanges 32, 34, 36and 38 and opposed longitudinal dividing ribs 40 and 42, respectively.The two sections 16 and 18 deline a furnace interior which is divided byribs 40 and 42 into two parallel, longitudinal extending heatingchambers A and B. It is to be observed that the two body sections are inparallel spaced relation with each other so that heating chamber'A isopen at edges 32 and 34 and heating chamber B is open at edges 36 and 38and both chambers are open at their ends. In addition, chamber Acommunicates with chamber B. Mounted in both chambers at the top andbottom thereof are longitudinally extending electrical heater rods 46and 48'which, when energized by a suitable source, serve to heat thechambers to desired temperatures. Chamber A and heater rods 46 togetherconstitute a source heater. Chamber B and heater rods 48 togetherconstitute a diffusion heater.

The edge flanges 32 and 34 are provided with identical grooves 50 whichserve as guide tracks for a plurality of movable ceramic carriages 54.Edge flanges 36 and 38 have identical grooves 52 for a like plurality ofidentical movable ceramic carriages 56.- Each carriage is of rectangularblock configuration having identical tongues 58 at two opposite sides, asimilar tongue 60 at a third side, and a slot 62 at the fourth side.Each slot 62 is sized to receive a tongue 60 of another seal 54. Thetongues 58 are sized to make a snug but slidable fit in grooves 56 and52. The carriages 54 and 56 are arranged in pairs, being attached toopposite ends of a plurality of identical open 7 ceramic carrier tubes66. The outer diameter of these tubes is slightly less than the distancebetween rigs and 42, being-just large enough to substantially block offany convection currents between chambers A and B but small enough to bemovable longitudinally along the space between the ribs.

The carriages 54 and 56 are partially encased in metallic collars 70 and72 which are connected to a pair of endless chains 74 and 76 byidentical bracket members 78 and'80, respectively. Chain 74.is mountedon suitable sprockets 84 and 86 attached to a pair of shafts 90 and 92journaled at opposite ends of the frame. Although not fully shown, it isto be understood that'chain 76 is nace are two guide rails 104 and 106which support the upper runs of chains 74 and 76. The vertical length ofthe brackets 78 and 80 is such that with the chains riding V on theguides 104 and 106, the carriages 54 and 56 are supported at theelevation required to allow their ribs 58 to ride in grooves and 52.

When the motor M is operating, the carriages and the ceramic carriertubes 66 will be driven by brackets 78 and 80 in an endless path in thedirection shown in FIGJ. Tubes 66 and carriages 54 and 56 will enter thefurnace at one end and will exit at the opposite end. During the travelthrough the furnace, the tubes will be supported in a horizontal planeby the carriages 54 and 56. The

' carriages 54 (and also the carriages 56) will engage each other whilethey are in the furnace. In this connection, it is to be observed thatthe brackets 78 and'80 are spaced along the chains 74 and 76 a distancesuch that the tongues on one carriage will be fully disposed within theslot 62 of the preceding carriage While the tubes 66 are moving alongwithin the furnace. When the ribs 60 are disposed within the slot 62,carriages 54 and 56 effectively seal off the spaces between flanges 32,34 and 36, 38, respectively, thereby completing the side walls of thefurnace. The side walls may be considered as made up of the flanges 32,34, 36, and 38 and the carriages 54 'and 56. On leaving the interior ofthe furnace, the carriages move away from each other as they travelaround sprockets 86 and re-engage each other after they have passedaround the sprockets 86. The spacing movement g. ceramic carrier tubes66 as described hereinafter. Loading and unloading is also facilitatedby the speed at which the chains travel. The chain speed is relativelyslow so as to allow tubes 66 to be within the furnace for a substantialinterval, -e.g., 22m 36 hours, depending upon the particular diffusionprocess to be executed.

The ceramic tubes66 function as' carriers for quartz heating tubes ofthe kind used heretofore in difiusion processes. These quartz tubes maybe of various types. For example, they may be fully sealed off, eitherpermanently or by removable end caps for so-called close tube operation.The permanent type is useable only once since it must be broken open inorder to remove its contents. Alternatively, the quartz tubes may also.be provided with end openings for so-called open tube operation whereina gas is circulated through the tubes to promote diffusion orout-diffusion. The quartz tubes 110 shown 'in FIGS. 1-4 are of thelatter type.

In practice, each quartz tube 110 functions to contain (l) a boat 112within which is a source of impurity 114 which is to be diffused into asemiconductor material and (2) a boat .116 within which is stacked aplurality of thin wafers or disks118 of a crystal semiconductor. Theboat 112 is disposed within the quartz tube 110 in a position such thatwhen the quartz tube is transported through the furnace by a carriertube 66, the source 114 will travel through chamber A. Boat 116 ispositioned so that it will travel through chamber B.

At this point, it is to be observed that the heater rods 46 arecontrolled (by means not shown) so that chamber A will have atemperature sufficient to cause vaporiza: tion of the source 114. On theother hand, the heater rods 48 are controlled so as to produce inchamber B a temperature at which the vaporized impurity will diff seinto the surfaces of the crystal wafers 118. In a typical case, chamberA will have a temperature of 300 C., and chamber 13 will have atemperature of 1200" C. As each quartz tube is transported along withinthe furnace, the impurity in boat 112 will vaporize, and then, due toits own vapor pressure or to the influence of a carrier gas, thevaporized impurity will envelope the crystalline wafers 118 and diffuseinto. their surfaces.

As seen, in FIG. 3, each quartz tube 110 is provided at its oppositeends with removable end caps 124 and 126 having small tubular extensionsto which may be connected flexible hoses 128 and 136, respectively. Asseen in FIG. 1, hoses 128 are attached to a suitable inlet manifold 132which is coupled by a tube 134 to as gas supply (not shown). Althoughnotshown, it is to be understood that flexible hoses 13% are connected toan outlet manifold of similar construction located on the opposite sideof the furnace. The function ofhoses 128 and and the manifolds to whichthey are attached'is to permit an appropriate inert gas such as argon tobe circulated through the quartz tubes (in the direction of the smallarrows in FIG. 5) at a suitable rate for the purpose of facilitatingdiffusion. The gas promotes distribution of the vaporized impuritythroughout the quartz tubes and also helps control its concentration,thereby helping to provide precise diffusion control. The hoses 128 and130 are attached to the quartz tubes immediately after they are insertedin the carrier tubes 66 but before the tubes enter the furnace. Asindicated in FIG. 5, the carrier gas is made to circulate as soon as thetubes enter the furnace and is cut off after they leave the furnace. Thehoses are removed after the tubes have left the furnace. T he'quartztubes are removed after they have been transported out of the furnacebut before passing through any substantial angle about sprockets 86.Because of the slow speed of the conveyor chains, the quartz tubes haveadequate time to cool before being removed from the carrier tubes.

Although the use of a carrier gas is most beneficial, the factor whichis primarily responsible for attainment of precise diffusion control isthe provision of the two parallel heater chambers A and B. With thisarrangement,

the source is vaporized at a relatively constant rate since it isexposed only to the temperature in heater chamber A which is maintainedsubstantially constant throughout its length. Moreover, diffusion of theimpurity into the crystal proceeds at an even, predictable rate and forthe time duration of the crystals travel through the furnace since thecrystal is exposed only to the temperature in chamber B which ismaintained substantially fixed throughout its length at the desiredlevel. Accordingly, the furnace will operate equally with closed tubes,in which case the flexible hoses 12S and 130 are not required. In thisconnection, it is to be noted that in any closed tube operation, thepartial pressure of the impurity itself promotes contact with thecrystal wafers, the vaporized impurity being present in sufficientquantity to assure super saturation of the surfaces of the wafers.However, as a consequence of the super saturation condition, it is oftennecessary to transfer the wafers to another furnace to executeout-diffusion. The latter may be defined as the process of boiling offexcess impurity from the wafers. In out-diffusion, it is customary topass an inert carrier gas over the heated crystal for the purpose offlushing away excess impurity as it is released by heating of thecrystal. A noteworthy advantage of the furnace shown in FIG. 1 is thatit may also be used for a straight out-diffusion process. Of course,such use does not require operation of the source heater. Forout-diffusion, hoses 128 and 130 will circulate the insert carrier gasthrough the quartz tubes in the same manner described previously inconnection with straight diffusion.

FIGS. 6 and 7 illustrate still other variations of the same invention.In FIG. 6, gas is circulated through the quartz tubes 110A continuouslywhile they are in the furnace. However, the source heater chamber A1 isillustratedas shorter than the diffusion heater chamber B1. At thispoint, it is to be noted that the two chambers may actually be ofdifferent lengths. Alternatively, they may have the same physicallengths, but the effective length of chamber A from a heating standpointmay be restricted by shortening the heater rods 46 so that theyterminate a substantial distance from the end of the furnace. As aconsequence, vaporization and diffusion of the impurity will bestoppedwhen the tubes reach a predetermined point along the length ofchamber B, and thereafter, since the wafers are still being heated inchamber B, excess impurity absorbed by the crystal will diffuse out andbe flushed away by the carrier gas. FIG. 7 illustrates how the furnacealso may be used for a double diffusion process. Here the furnace isprovided with two source heater chambers A2 and A3 and a singlediffusion heat chamber B2. Each quartz tube 110B contains three boats,two of which contain two different sources of impurity 114A and 11413and the third of which contains crystal wafers 118A to be doped. Becausethe effective upstream end of chamber A3 is downstream of the upstreamends of chambers A2 and B2, source 114A will vaporize and start todiffuse before source 114B, the latter will vaporize and start todiffuse before diffusion of impurity from source A2 has ceased.Out-diffusion occurs with the cooperation of the carrier gas for aprescribed period of time after diffusion of impurity from source 1143has ended.

Of course, it is not necessary to carry out double diffusion preciselyas described in connection with FIG. 7. Thus, for example, chambers A2and A3 could be arranged so as to start vaporization of sources 114A and1143 at spaced intervals or substantially simultaneously and toterminate them simultaneously or in prearranged time sequence.

It is to be noted that it is not necessary to fully seal off the sidesof the heater chambers in the manner accomplished by the carriages 54since the amount of heat which can be lost through the openings thereinis not great. These openings are not large, the outer diameter of thecarrier tubes 66 usually being in the order of 2 /2 inches. Moreover,simple inexpensive radiation shields may be invention is not limitedpositioned along the furnace sides to minimize heat loss.

The present invention has many important advantages. For one thing, itmakes possible furnaces which can be used for single or multiplediffusion and also for out-diffusion, either as a separate operation oras the final phase of a diffusion process. With respect to diffusion, itis to be noted also that different quartz tubes may have differentsources of impurities and also different kinds of semiconductormaterial, whereby different products may be produced in the samefurnace. Moreover, furnaces constructed as described above are capableof producing junction layers of precisely controlled thicknesses, whileat the same time achieving greater production than is possible in batchfurnaces. Further advantages of the foregoing type of construction arethat it is capable of being built with existing materials and utilizingconventional quartz tubes. Also important to note is that the life ofthe heating elements is much greater than it is in the case of a batchfurnace since it is not necessary for furances of the type described tobe repeatedly cooled down as are batch furnaces. It is also recognizedthat the invention may be utilized for processes wholly unrelated to themanufacture of semiconductors.

Obviously, many modifications and variations of the present inventionare possible in the light of the above teachings. It is to beunderstood, therefore, that the in its application to the details ofoperation or of construction and arrangement of parts specificallydescribed or illustrated, and that within the scope of the appendedclaims, it may be practiced otherwisethan as specifically described orillustrated.

I claim:

1. A furnace adapted for use in the manufacture of semiconductors bythe, process of gas diffusion comprising means defining two parallelelongated furnace chambers each open at both ends and also having arestricted opening at a side immediately adjacent to the other, meansfor'heating one chamber to a first temperature, means for heating theother chamber to a second temperature, a series of hollow heating tubeseach adapted to hold a semiconductor material and a vaporizable impuritywhich is to be diffused into said semiconductor material, and means foradvancing said series of hollow'heating tubes in succession. throughboth chambers simultaneously whereby said semiconductor material isheated in said one chamber and said impurity is vaporized in said otherchamber.

2. A furnace as defined by claim 1 wherein said heating tubes areremovably disposed in ceramic carrier tubes forming part of saidadvancing means, said carrier tubes substantially closing off theopening in said adjacent side of said each chamber as they pass throughsaid furnace whereby to prevent transfer of heat from one chamber toanother by convection currents.

3. A furnace as defined by claim 1 wherein said chambers are open attheir outer sides and said hollow heating tubes have their endsprojecting through said outer sides.

4. A furnace as defined by claim 1 further including means connected tosaid heating tubes for circulating a gas through each heating tubeadvanced through said chambers by said tube-advancing means.

5. A furnace as defined by claim 4 wherein said gas circulating meanscomprises a plurality of flexible hoses releasably connected to oppositeends of said each heating tube and stationary manifold means fordirecting gas into certain of said hoses and receiving gas from other ofsaid hoses.

6. A furnace as defined by claim 1 wherein said lastmentioned meanscomprises a pair of conveyor chains, a plurality of carriages carried byeach chain, and a series of ceramic carrier tubes extending between saidchains, said carrier tubes attached at their opposite ends to carrierson each chain, each heating tube removably disposed in one of saidcarrier tubes with each carrier tube having at least one end open topermit a heating tube to be inserted therein, said carrier tubessubstantially closing off the open adjacent sides of said chamberswhereby to minimize transfer of heat from one chamber to another so asto maintain said chambers at said first and second temperatures. I

7. A continuous gas diffusion furnace comprising means defining twoparallel elongated furnace chambers and a'passageway connecting saidchambers for the full length thereof, means for heating said sections todifferent temperatures, a plurality of hollow heating tubes, means forsupporting said plurality of heating tubes. in transverse relation tosaid elongated chambers, means for transporting said tube supportingmeans along the full length of said chambers whereby a portion of eachof the,

tubes supported thereby will advance through one of said chambers whileanother portion of said each tube will advance through the other chamberand an intermediate portion of said each tube will advance along saidpassageway, and' means connected to said heating tubes for flowing a gastherethrough during transport thereof through said chambers.

8. A furnace adapted for use in the manufacture of semiconductors by theprocess of gas diffusion comprising means defining two parallelelongatedfurnace chambers separated from one another by an intermediatewall member, each chamber open at both ends and communicating with theother by an elongated opening in said wall member, means for heating onechamber to a first temperature, means for heating the other chamber toasecond temperature, a series oftceramic carrier tubes each having atleast one end open, an endless conveyor having a run extending from oneend to the other of said furnace, means securing said carrier tubes tosaid conveyor in transverse relation to said run with a portion of eachtube aligned with one of said chambers and the remainder of each tubealigned with the other of said chambers, means for operating saidconveyor to advance said tubes in succession through both chamberssimultaneously with each carrier tube entering said chambers at one endand leaving said chambers at the other end, and a plurality of hollowheating tubes each removably disposed in one of said carrier tubes,suficient to simultaneously accommodate and transport a semiconductormaterial and a vaporizable impurity which is to be diffused into saidsemiconductor material with said semiconductor material positioned to betrans ported through said one chamber and said vaporizable said heatingtubes having a length a impurity positioned to chamber. a

9. A furnace as defined by claim 8 wherein said carrier tubessubstantially close off said elongated opening as they pass through saidfurnace whereby to prevent transfer of heat from one chamber to theother so as to maintain said chambers at said first and secondtemperatures. 10. A furnace as defined by claim 8 wherein said chambersare open at theirouter sides and said carrier tubes have their endsprojecting through said outer sides, and further wherein said carriertubes are connected to said conveyor at their projecting ends.

11. A furnace as defined by claim 8 wherein said conveyor comprises apair of parallel conveyor chains disposed outside of said chambers and aplurality of like carriages carried by said chain having a counterparton the other chain, and further wherein said carrier tubes are securedto said carriages. 12. A furnace as defined by claim '11 wherein saidchambers are open at their outer sides, and further where- 'in saidcarriages travel along in the planes of said outer sides.

13. A furnace as defined by claim 12 wherein said carriagessubstantially seal oh the openings in said outer sides.

14. A furnace as defined by claim 8 further including means connected'tosaid heating tubes for circulating a gas through each heating tube as itadvances through said chambers. 7

References Cited by the Examiner UNITED STATES PATENTS 1,005,335 10/11Seigle 263-8 1,531,214 3/25 ODonovan 263-37 1,549,880 8/25 Johnson263-37 1,738,039 12/29 ,Cope et a1 263-36 X 1,926,354 9/33 Spatta 263-82,588,141 3/52 McFarland et a1. 263-8 2,683,652 7/54 Martin 23-277 X2,887,453 5/59 Billig et a1. 252-623 2,921,905 1/60 Chang 252-6233,078,082 2/63 Hnilcka 266-19 CHARLES SUKALO, Primary Examiner. JULIUSGREENWALD, JOHN J. CAMBY,'Examiners.

be transported through said other chains, each carriage on one

7. A CONTINUOUS GAS DIFFUSION FURNACE COMPRISING MEANS DEFINING TWOPARALLEL ELONGATED FURNACE CHAMBERS AND A PASSAGEWAY CONNECTING SAIDCHAMBERS FOR THE FULL LENGTH THEREOF, MEANS FOR HEATING SAID SECTIONS TODIFFERENT TEMPERATURES, A PLURALITY OF HOLLOW HEATING TUBES, MEANS FORSUPPORTING SAID PLURALITY OF HEATING TUBES IN TRANSVERSE RELATION TOSAID ELONGATED CHAMBERS, MEANS FOR TRANSPORTING SAID TUBE SUPPORTINGMEANS ALONG THE FULL LENGTH OF SAID CHAMBERS WHEREBY A PORTION OF EACHOF THE